We have recently cloned a glucose transporter from brown trout muscle (btGLUT) with high sequence homology to mammalian GLUT4 that is predominantly expressed in red and white skeletal muscle, the two major sites of glucose uptake in trout. To study the physiological regulation of this putative fish GLUT4, we have investigated the expression of btGLUT in red and white skeletal muscle of trout in which blood insulin levels have been altered experimentally. The expression of btGLUT in red muscle increased significantly when insulin plasma levels were elevated by either insulin or arginine treatment and decreased significantly when insulin plasma levels were reduced either by fasting or by feeding a low-protein, high-carbohydrate diet. In contrast, the expression of btGLUT in white muscle was not affected by changes in the plasma levels of insulin. These results strongly suggest that insulin could be regulating the expression of btGLUT in trout red muscle in vivo and set the ground to test the hypothesis that btGLUT may be considered a GLUT4 homolog in fish.